Field of the Invention
The present invention relates to a method and an apparatus for computing a rotation angle of a rotor in a motor.
Brief Description of the Related Art
FIG. 5 shows the arrangement of a conventional brushless direct current motor 100. The brushless direct current motor 100 has a rotor 50 that is implemented with a permanent magnet and rotatably held within a stator 80. The stator 80 has a multiplicity of coils 85 controlled by an electronic circuit 90 in order to generate a magnetic field within the stator 80. The rotating field causes a torque on the rotor 50. The commutation of the direct current motor 100, i.e. the switching on and off of the coils 85, is effected via the electronic control 90 and is usually dependent on the rotor position, the rotational speed and the current load of the rotor 50. Angular errors upon measuring the rotor position of the rotor 50 lead to high power dissipation in the direct current motor 10, consequently reducing the available maximal torque. These angular errors are frequently also the causes of noise and vibrations in the direct current motors 100.
For the correct measurement of the rotor position the direct current motors 100 must be calibrated. This calibration is initially effected in a known manner by external sensors, i.e. Hall sensors, placed in the front field of the motor 100 and with a permanent magnet fixed to the rotor 50.
In the state of the art also patent publications are known which disclose sensors for capturing the rotor position in a direct current motor. For example, the Japanese patent application No. JP 2005/308430 of the Matsushita Electric company teaches a contactless detector for the rotation angle which can be used in the direct current motor.
However, these known solutions do not permit a calibration of the sensor during operation. Such a calibration would permit both greater mounting tolerances in the manufacture of the motor and positional changes of the motor or of the sensors during operation. Moreover, an automatic calibration enables a continuous compensation of static and dynamic interference fields in the vicinity of a running motor, which otherwise influence the measurement of the rotation angle.